My first post was just about some ideas. Since I didn't know any more than your initial pitch I figured you were still building up the story.
As for how exactly, if you can't somehow suggest the player into doing things there's really no way to expect they'll do it.
Another psychological trick, tell the players it's a game about killing, advertise it as one (http://en.wikipedia.org/wiki/Observer-expectancy_effect). They will have expecations because of your bias towards it before they begin playing. They already expect to be a killer before they begin to play.
From what you wrote, maybe your target audience are players who want to vent anger. Hopefully they'll understand by the end that their actions were an extension of their own willpower and ingenuity. If they ever stop to think about it.
I may have accidentally made the assumption you didn't want some players to have a realization mid-way when I wrote:
To reduce the severity, you shouldn't actually show the killing until the end, if the player's annoyed enough they'll think about it.
4. Come again?
I'll just put this in terms of writing for the moment. If you described the gore and screams of your victims in every scene the sensitive folk who are talked into participating will be revolted and reset the game. It wouldn't take long before no one but the strangest players keep on going. So if you really plan on tricking people, I wouldn't give them the blow by blow, and they'll supply their own "it's just a movie" mantra to see what's next.
Does that make more sense?